4 What Happens When Compounds Are Added to Water?

Transcription

1 4 What Happens When Compounds Are Added to Water? Name: Date: Lab Day/Time: Lab Partner: Objectives Understand the difference between electrolyte and nonelectrolyte solutions Learn to identify a compound as covalent or ionic and predict its conductivity in solution Learn to write chemical equations for compounds that dissociate into ions in solution Understand the relationship between moles of ions in solution and conductivity Pre- Laboratory Requirements Watch the video on Canvas titled Using a Conductivity Meter Pre-lab questions (if required by your instructor) Laboratory notebook prepared before lab (if required by your instructor) Safety Notes Eye protection must be worn at all times. Discussion In this experiment, you will explore the conductivity of electrolyte and nonelectrolyte solutions. Compounds that are electrolytes dissociate into ions when dissolved in water. When electrodes are immersed into an electrolyte solution, the ions complete the electrical circuit and the solution conducts electricity. Nonelectrolytes do no dissociate into ions and, therefore, these solutions do not conduct electricity. In this lab, the conductivity of solutions will be measured with a Relative Conductivity Indicator. Procedure Part I. What Happens When You Put Chemical Compounds in Water? One way you can explore the characteristics of different solutions is to measure whether or not they conduct electricity and to what extent. Using the Conductivity Tester: To measure the amount of electricity that your solution conducts, you will be using a Conductivity Meter. When you are ready to make a measurement, press the on/off button on the meter and place the probe in ~ 20 ml of deionized water in a beaker. Make sure the meter is reading close to zero (0-3 µs) before you place it into your solutions. Record the conductivity reading and units (ms or µs) then remove the probe from your solution. Thoroughly rinse the probe with deionized water and then place it back into the beaker of deionized water to ensure the meter is reading close to zero between each measurement. CAUTION: Do not touch any other buttons on the conductivity meter except the on/off button James Madison University and JMU Board of Visitors

2 1. Investigate the properties of deionized water. Do NOT use regular tap water. Test the conductivity of the deionized water with the conductivity meter, record the reading and then immediately remove the probe from the water. 2. Prepare solutions of calcium chloride, dextrose, isopropyl alcohol, sodium chloride, and sucrose by adding approximately 0.15 g of each compound in 30 ml of deionized water. As you put each of your compounds into water, stir and carefully observe what happens. Record all of your observations. Be sure to label each of your solutions. 3. Using the conductivity meter, investigate the electrical conductivity of each solution and record the reading with units (ms or µs). Between each test, be sure to rinse the conductivity meter with deionized water. Compound Observations Chemical Formula Conductivity reading Conductive? Nonconductive? Deionized Water Calcium Chloride Dextrose Isopropyl Alcohol Sodium Chloride Sucrose 4. (Optional) Compile class data to investigate the reproducibility of the measurements. Do the trends observed by your group correlate with the class data? Compound AVG Deionized Water Calcium Chloride Dextrose Isopropyl Alcohol Sodium Chloride Sucrose James Madison University and JMU Board of Visitors

3 Interpreting the Data Discuss your answers to the following questions with your lab partner(s) and record them: 1. What types of changes did you observe as you prepared your solutions? 2. How did the different solutions behave when you tested them with the conductivity meter? What relationship did you observe between the chemical formulas and conductivity? Summarize your conclusions below. 3. How would you represent your observations with balanced chemical equations? Compound + H 2 O Balanced Chemical Equation Calcium Chloride Dextrose Isopropyl Alcohol Sodium Chloride Sucrose Class Discussion What happens to chemical compounds when you put them in water? Be prepared to explain your current thinking and the observations that led you to your ideas. You may find it helpful to take notes about the discussion. Part II. Predicting How New Systems Will Behave Predict which compound will produce the largest number of pulses, i.e. be the most conductive, when dissolved in ml of water. Circle your prediction g NaCl 0.10 g NaI 0.10 g CaCl 2 all will produce the same # Provide a brief explanation for your prediction James Madison University and JMU Board of Visitors

4 1. Prepare solutions of NaCl, NaI and CaCl 2 by adding 0.10 g of each solid to a different ml volumetric flask. Fill each flask with deionized water such that the bottom of the meniscus touches the mark denoting ml. Place a stopper on the top of each flask and mix well by inverting the flask. 2. Pour a sample of each solution into a labeled beaker. Using the conductivity meter, test the conductivity of each solution and record the reading. Between each test, be sure to rinse the conductivity meter with deionized water. Compound Mass (g) Moles (mol) Volume (ml) Volume (L) Conductivity reading NaCl NaI CaCl 2 3. (Optional) Compile class data to investigate the reproducibility of the measurements. Do the trends observed by your group correlate with the class data? Compound AVG NaCl NaI CaCl 2 4. Calculate the molarity of each solution and the concentration of ions in each solution. Record your results in the table below. Compound Conductivity Reading (Group or Class AVG) Concentration of salt solution (M) Concentration of ions in solution (M) NaCl NaI CaCl James Madison University and JMU Board of Visitors

5 Sample Calculations: Interpreting the Data Discuss your answers to the following questions with your lab partners and record them: 1. Describe, in your own words, the difference between the concentration of the salt solution and the concentration of ions in solution. 2. What determines the magnitude of the conductivity? 3. How did the conductivity values compare for these three solutions? How can you explain your observations? Class Discussion Have your observations in Part II changed your thinking compared to Part I? Be prepared to explain your current thinking and the observations that support your ideas James Madison University and JMU Board of Visitors

6 Part III. Why Does Your Source Of Water Matter? This lab has been very specific about the type of water you should use. It emphasized the importance of using deionized water, rather than tap water, for all experiments. In the final portion of the lab, you will investigate why this decision was made. Predict what will happen if you use the conductivity probe with tap water. Explain the rationale behind your prediction. 1. Test tap water with the conductivity indicator and record the reading. Conductivity reading Tap Water Interpreting the Data 1. What do your results tell you about tap water? 2. Why did you need to use deionized water rather than tap water? James Madison University and JMU Board of Visitors